Please keep in mind that in Islam, regardless of whether you're a hijabi or not, it's Haram for a woman to show her legs and wear fitted clothing (in public. of course you are given more freedom once married, and if you need more guidance on that, then there are some hadiths and...
To perform prayers wearing a singlet for a man is not in compliance with the seriousness of the prayer. It does not invalidate the prayer but it is more suitable to wear a shirt on it and pray. (see Mehmed PAKSU, İbadet Hayatımız) Questions on IslamAuthor...